Bolometers are thermally isolated detectors that are heated by infrared radiation absorbed on their surface. The temperature rise is converted to an electrical signal using a thermistor element. Infrared bolometer detectors are widely used for thermal targeting, night vision systems, and motion sensing. The most sensitive bolometers available are cryogenically cooled superconductive devices. Often their cooling system is heavy and bulky; hence these devices are not adequate for lightweight and portable systems. In addition, the cryogenic cooler is the most expensive component in the photon detector IR camera, and has finite lifetime only around 2,000 hours.
Recent work by Honeywell and Texas Instruments has demonstrated the feasibility of uncooled bolometric devices. Honeywell's device used a micromachined array of microbridge-type bolometric pixels 50.times.50 .mu.m.sup.2 each. The temperature detector was a VO.sub.x resistor patterned on the microbridge which has a sharp transition in its resistance near room temperature. Similar technology has been applied into different developed commercial available infrared imagers. The Texas Instruments device uses a pyroelectric pixel of barium strontium titanate (BST) that is patterned directly on the substrate. Both devices have a backplane of interfacing electronics for multiplexing of the pixel signals. Recently, the research work has been concentrating on the monolithic fabrication of infrared detector using micromachining techniques which trade the fill factor with potential high yield and lower production cost.
In all of these technologies, the infrared detectors have highly process dependent responsivities and offsets requiring elaborate calibration procedures and the extensive use of corrective electronics.
Most conventional bolometers operate in the open loop scheme shown in FIG. 1. Upon the absorption of incident radiation, the temperature of a thermally isolated plate or absorber 10 increases changing the electrical characteristic of a thermistor 12. The thermally isolated plate 10 defines a thermally isolated area. The electrical signal of the thermistor 12 is typically amplified yielding voltage V.sub.o representing the power of the incoming radiation. One of the major problems with this mode of operation is that the measured signal is influenced by many material properties, offsets and drifts which make the response difficult to predict.
For example in the simple open loop detector circuit shown in FIG. 2, the bolometer R.sub.d with temperature coefficient of resistance .alpha. is connected through biasing resistor R.sub.1 to the voltage source V. For incident power .DELTA..PHI., the bolometer temperature increase .DELTA.T.sub.d obeys: ##EQU1##
where:
C=the heat capacitance of the bolometer element; PA1 G.sub.0.DELTA.T.sub.d =the conductive and radiative heat flow for the element; and PA1 W.sub.h =the self-generated thermal power in the bolometer. PA1 G.sub.e : effective thermal conductance; PA1 G: thermal conductance for a small temperature change; PA1 .epsilon.: the emissivity of the bolometer; and PA1 .omega.: sinusoidal radiation input frequency.
The responsivity of this detector circuit is: ##EQU2##
where ##EQU3##
and
The responsivity thus depends on the material properties like .alpha., and R.sub.d that vary from run-to-run hence a calibration procedure is needed. In a focal plane imaging array, this must be done for each pixel in the array.